In digital communication systems that support several modulation and coding schemes (MCS), the transmitter may have a method for dynamically selecting MCS. The problem of dynamic MCS selection is of special interest in wireless communication systems, where the channel may significantly change over time. For simplicity, the term MCS is used throughout this application to encapsulate the combination of modulation and coding scheme (MCS) as well as sampling rate, bandwidth, number of spatial streams, etc. Two distinct combinations are considered as two different MCS even if they result in the same number of data bits per second.
Typically, it is desirable for a transmitter to work at, or close to, the maximum possible transmission rate rmax of the channel instance, in order to maximize the system throughput. However, working near the maximum transmission rate may be risky. In typical wireless systems, choosing a transmission rate above rmax may result in high packet error rate (PER), which in turn may result in an undesired goodput loss.
Many current MCS selection methods fall in the category of PER-based trial and error. Basically, assuming that the transmitter has some short term estimation of PER, MCS is changed such that the transmission rate increases if PER is low enough and changed such that the transmission rate decreases if PER is too high. A more sophisticated method of this type is known as the Auto Rate Feedback. While simple, these methods have relatively poor performance because of their slow settling time, and because they may require long high-PER phases for convergence.
Fast link adaptation methods require that the transmitter have some knowledge of dynamic channel quality information (CQI), e.g., an estimation of signal to noise ratio (SNR) or an effective SNR, for MCS selection. To that matter, CQI may also refer to quantized SNR, or even to MCS recommendation feedback from a receiver, such as that of the fast link adaptation (FLA) mechanism of the IEEE 802.11n standard. Each supported MCS is related to a level of throughput and has a minimum required channel quality. The transmitter chooses the MCS with the highest throughput for which the current quality is above the minimum required quality. Clearly, the performance of fast link adaptation methods relies, inter alia, on the accuracy of the estimated channel quality. If, for example, SNR is used as CQI, then an error in order of 1-2 dB in the estimation of SNR may dramatically increase PER.